The present invention relates to a projection exposure system, and more particularly, to a transformed illumination system of a projection exposure system used in a photolithography process.
The primary technique for acquiring fine patterns for liquid crystal displays or semiconductor devices is photolithography. When using photolithography, projection exposing technology affects the packing density and quality of the resulting semiconductor devices.
An excimer laser stepper, which reduces the wavelength of the light source used, a phase shift mask, and transformed illumination such as tilted illumination have been used to improve projection exposing technology.
Transformed illumination has been of great interest because it improves resolution without new equipment. Conventional transformed illumination technology for use in a projection exposure system will be explained with reference to FIGS. 1, 2 and 3.
The projection exposure system illustrated in FIG. 1 that provides transformed illumination has a filter 3 attached to a conventional illuminator containing a light source 1, a fly's eye lens 2, a condenser lens 4 and a mask 5. The filter is constructed as shown in FIGS. 2 and 3, with FIG. 2 illustrating an annular illumination filter and FIG. 3 illustrating a four-point illumination filter. In FIGS. 2 and 3, hatched portions indicate light blocking portions and unhatched portions indicate light transmitting portions.
In the annular conventional illumination filter that provides tilted illumination, light transmittivity is (.sigma..sub.o.sup.2 -.sigma..sub.i.sup.2)/.sigma..sub.o.sup.2. Usually, since the most appropriate value of .sigma..sub.i is (2/3).sigma..sub.o, the transmittivity becomes five ninths and the exposing time is approximately double that when the transmittivity is one, resulting in greatly reduced productivity.
In the case of four-point illumination, the transmittivity is 4.sigma..sub.i.sup.2 /.sigma..sub.o.sup.2. Here since .sigma..sub.i =(1/4).sigma..sub.o, the transmittivity becomes one fourth and the exposing time is approximately quadrupled, to further deteriorate the productivity.